1. Field of the Invention
The invention is related to the field of microdischarges or plasmas in a tube geometry and in particular to direct current discharges at relatively high pressures using metal tubes which are operated as the cathode and using a metal grid or plate as the anode.
2. Description of the Prior Art
Hollow cathode microdischarges have gained recent attention due to their high pressure operation and intense UV radiation. The discharges are characterized by higher current densities ( up to 10 A/cm2) at lower operating voltages in comparison to conventional glow discharges at similar conditions. Furthermore, optical studies have shown the presence of highly excited states such as neon ions more than 50 eV above ground states and excimers. Therefore, it is believed that these discharges contain a large concentration of high-energy electrons making them potentially useful as UV lamps and plasma reactors.
While various electrode geometries have been explored to take advantage of the hollow cathode, in general, a thin metal plate less than 200 xcexcm thick with an aperture between 100-700 xcexcm in diameter serves as the cathode. The pressure at which the discharge can be operated has been shown to depend inversely on the hole diameter with atmospheric-pressure operation requiring diameters less than 250 xcexcm in rare gases and less than 100 xcexcm in air. Devices in most of these cases consist of a metal-dielectric-metal structure with a hole through all three layers. Recently, the structure has also been expanded to multilayer structures in order to increase the active length of the device. Lifetime and stability of microdischarges in this configuration are limited by the dielectric, often a polymer, which can fail due to deposition of sputtered cathode material and thermal decomposition. Because of these concerns, discharge currents are often kept below 7 mA to extend the lifetime of devices. Multilayer structures also suffer from complex fabrication steps with only small increases in the total length of the device.
Hollow cathode microdischarges in the prior art are stable, high-pressure discharges formed between a cathode with a hole and an anode of arbitrary shape. It has been previously found experimentally that it is necessary to reduce the cathode hole diameter to near 100 xcexcm to allow operation at atmospheric pressure in rare gases such as neon, argon, and xenon. The electrode geometry usually consists of a sandwich structure of two metal plates on either side of a thin dielectric spacer. Discharges are struck in the confined volume between the metal electrodes in a direct current mode with similar voltages used for conventional glow discharges, but much larger current densities.
The increase in the number of ionization processes is caused by the Pendel effect, which is the oscillatory motion of electrons in the radial electric field created by the hollow cathode. Optical studies in rare gases have confirmed the presence of a large concentration of high energy electrons by the emission of excimer radiation and other highly excited states. These properties warrant the use of microdicscharges in materials processing where the production of reactive radicals at high pressures is often required. We have recently reported one such application where Ar/CF4 microdischarges were used to etch silicon.
Tubes have been simultaneously used in the prior art as the gas inlet and cathode, but with openings of the order of 0.4-2 mm, which are much larger than those found in hollow cathode microdischarges. For this reason, the discharges were operated at lower pressures (p less than 1 Torr) and used radio frequency power which requires complicated impedance matching networks. Furthermore, in some cases, although operation was achieved at atmospheric pressures, the discharge was found to form on the surface of the electrodes and did not operate as a hollow cathode.
Further, such hollow cathode microdischarges have a flat or disk geometry in which the plasma is confined to the small disk-shaped space between opposing dielectric planes. This geometry excludes its usage in many applications where a projecting plasma onto a material substrate is needed. What is needed is some kind of method and apparatus having a geometry whereby hollow cathode microdischarges can be effectively and practically extended to interact with surfaces.
An alternative concept to increasing the length of the cathode from that used in hollow cathode microdischarges is realized by forming multilayer structures to extend the hollow cathode to a tube geometry. This approach increases the length of the cathode by orders of magnitude. Furthermore, producing a discharge in a flow geometry would be more conducive for applications in air, such as gas detoxification and spectroscopy. Similar tube geometries have been previously used to operate atmospheric-pressure plasmas, but due to larger openings (0.4-2 mm) were observed to have surface discharge formation. The objective of the present invention is to show stable DC operation of a hollow cathode discharge at atmospheric-pressure in metal or conductive capillaries with openings less than 250 xcexcm in diameter.
In a preferred embodiment of the invention it assumes a geometry in which microdischarges can be utilized as a radical source by providing a flow or jet where species produced in a hollow cathode are transported to a substrate. In order to obtain hollow cathode operation at high pressures or at least operation at atmospheric or subatmospheric pressures, further shrinking of the hole diameter is necessary, similar to that used for microdischarges in metal plates. The ability to form microdischarges using direct current bias in a flowing environment takes advantage of the properties of a hollow cathode and is advantageous for film deposition. In the illustrated embodiment flowing discharges in metal capillary tubes with hole sizes as small as 178 xcexcm are described and used for the deposition of diamond films.
However, it must be expressly understood that the hollow cathode plasmas or flowing discharges of the invention can be used for any application and hole sizes of the tubular cathode may assume any value within a range of diameters consistent with the teachings and spirit of the invention.
In one embodiment, the plasma microjet is comprised of a stainless steel capillary 5 cm in length with a hole diameter of 178 xcexcm. The capillary, operated as the cathode, was separated from a metal screen, which served as the counter electrode or anode. The screen was positioned by a linearly movable micrometer stage, which allowed for control of the distance between the cathode and anode. A negatively biased DC power supply operates the discharge with a current-limiting resistor (Rc) in series with the microjet. Gases such as argon and helium were flowed through the capillary using a mass flow meter with rates between 100-500 sccm. After the discharge was initiated, the plasma current-voltage (I-V) was monitored by measuring the voltage across resistors in series and parallel with the plasma. Current instabilities on short time scales were also observed using a digital oscilloscope. Argon discharges were characterized by optical emission spectroscopy using a SPEX 1680 double monochromator and a Hamamatsu photomultiplier tube model no. R928.
Breakdown voltages of the hollow cathode microjet depended inversely on the distance between the end of the capillary tube and the screen. Reducing this gap to less than 0.5 mm permitted breakdown of the gas at voltages less than 1000 V. After the discharge was initiated, the screen could be moved to extend the length of the discharge outside the tube. The appearance of the microjet was found to depend on both the gas flow rate and the distance between the cathode and anode (L). As the distance increased, the minimum current required to sustain the plasma increased with the plasma extinguishing below this value. The operating voltage of the discharge increased from 280 V to 400 V as the distance increased from 0.5 to 2.5 mm. The relationship between the sustaining voltage and distance is a super linear increase in voltage with distance. In the range of distances and current values studied, the microjet could be sustained with relatively few fluctuations and good stability over extended time periods of operation. Due to the stability of the discharge at low currents and voltages, it is believed that under these conditions the discharge behaves similar to a hollow cathode microdischarge. At higher currents and voltages, the fluctuations and instability may be the result of a transition of the plasma from a glow-like state to an arc.
More specifically, the invention is defined as an apparatus for combination with a source of gas comprising a conductive hollow elongate conduit or tube means, serving as a cathode, having a longitudinal axis and an exit orifice; a voltage source or means for providing direct or low frequency current having a negative terminal electrically coupled to the conductive hollow elongate conduit; and an anode or anode means electrically coupled to the voltage source and positioned at least at one point in time longitudinally distanced from the exit orifice of the conductive hollow elongate conduit. The voltage source typically operates at between 500 to 1500 volts depending on the desired plasma intensity. The source of gas is communicated with the conductive hollow elongate conduit to supply gas to the conductive hollow elongate conduit, so that upon application of the voltage to the conductive hollow elongate conduit a plasma is formed at least within the conductive hollow elongate conduit. In the preferred embodiment the anode is grounded, but in general there only need be a sufficient potential difference between the cathode and anode to strike a plasma. The source of gas may also be considered to be included as part of the apparatus in some embodiments or may be treated separately.
The source of gas provides a flow of gas through the conductive hollow elongate conduit so that a microjet of the plasma extends from the exit orifice and wherein the anode is downstream from the exit orifice. Even in the case where there is no flow of gas in the conduit or tube, the source of gas provides gas to the conductive hollow elongate conduit so that the plasma extends to the exit orifice.
In the preferred embodiment the conductive hollow elongate conduit is a metal tube, or more particularly a stainless steel tube. However, any conductive material may be used which is stable to the particular gas chemistry. Again, in the preferred embodiment the conductive hollow elongate conduit is a cylindrical tube with an inner diameter of approximately 200 xcexcm or less.
The preferred form of the anode is a conductive plate, grid or screen, which is movable or removable at least in part. Again the invention is not limited to this form of the anode which may take many other equivalent forms, including multiple part anodes, which include both stationary and movable portions. In many applications the anode will be formed by a work piece, such as a MEMS device, the nature of which will be determined in each by the application.
The invention further comprises a plurality of conductive hollow elongate conduits, each having a longitudinal axis and an exit orifice, where the voltage source has its negative terminal electrically coupled to each of the conductive hollow elongate conduits, and where the anode is positioned at least at one point in time longitudinally distanced from the exit orifice of each of the conductive hollow elongate conduits. For example, the anode may be fabricated so that a hole or ring is defined in or by the anode, which hole or ring surrounds and is aligned with the primary gas flow exiting from the tubular cathode. This may take the form of a hole in an anode plate or a wire anode ring through which the gas jet or flow from the tubular cathode is directed.
In the preferred embodiment the source of gas is a source of an inert gas, such as helium, neon, argon, or xenon and operates at atmospheric pressures in air or in a chamber filled with a selected gas. However, it is to be understood that reactive gases may be substituted according to the desired application.
The plasma formed by the apparatus in the illustrated embodiment is an efficient source of ozone, may be used as a ozonator, and is also a good source of ultraviolet emissions. In the illustrated embodiment because of the resistance of the stainless steel tubes to oxidation damage, the apparatus is capable of operating continuously in excess of at least 100 hours without replacement of the conductive hollow elongate conduits or tubes.
The invention is also defined as a method comprised of steps for performing the forgoing functions, namely a method comprising the steps of providing a conductive hollow elongate conduit having a longitudinal axis and an exit orifice; providing an anode electrically coupled to the voltage source and positioned at least at one point in time longitudinally distanced from the exit orifice of the conductive hollow elongate conduit; applying a negative voltage of direct or low frequency current to the conductive hollow elongate conduit; and supplying a gas to the conductive hollow elongate conduit, so that upon application of the negative voltage to the conductive hollow elongate conduit a plasma is formed at least within the conductive hollow elongate conduit.
While the apparatus and method has or will be described for the sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 USC 112, are not to be construed as necessarily limited in any way by the construction of xe2x80x9cmeansxe2x80x9d or xe2x80x9cstepsxe2x80x9d limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 USC 112 are to be accorded full statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals.